I too have Wagner 700 series helm pumps. Last year one started to leak at the shaft seal. I just drained the system at the cylinder, removed the steering wheels, and pried the seals right out of the front. Then I cleaned the recess and tapped the new seals into place using a piece of plastic tubing/pipe which slid easly over the shaft. There was some looseness in one of the shafts but not bad for 20 years use and I decided to see if the new, more flexiable, seal would cure it (otherwise the housing needs to be bored out and a bushing installed).
Over the years since I installed this system I have used both hydraulic jack oil (which is SAE 10) and ATF fluid w/ no problems except both made the steering heavy in the winter. So I decided to try something lighter. This time I used Arctic Blue hydraulic snowplow fluid. This is great stuff, thin as water, good lubricating properties, harmless to seals, & available at Advance Auto Parts and others. Steering much easier & smoother, no rattleling noise from pump, blue dye to help spot leaks, autopilot likes it better too.
Happy camper. Wagner was great stuff. The Wagner in my Willard is 51 years old and still going strong.